1. Field of the Invention
The present invention relates to a memory system having a memory extend function, which is used, for example, in a computer system such as a lap-top type personal computer.
2. Description of the Related Art
A conventional computer system of this type uses a standard memory having a memory capacity of, for example, 512 KB (Kilobytes) or 640 KB. Normally, the memory capacity required in an operating system (OS) is 512 KB or 640 KB.
A memory having a memory capacity of 1 MB (megabytes) has recently been used as a standard memory. In the case of the standard memory of 1 MB, 512 KB or 640 KB is used for the OS, and the remaining 512 KB or 384 KB is reserved for a BIOS (basic input/output system) RAM (random access memory), etc. The memory region used for the OS is referred to as "conventional memory", and the memory region used for the BIOS RAM, etc. is referred to as "reserve region".
There is known a method of effectively using the reserve region in a standard memory. According to this method, a memory region is extended by using an extended memory which can be freely mounted in a memory system. In this case, the address region of the reserve region is assigned to an address region of the extended memory. In other words, the reserve region is used as "EXTENDED memory" to extend the memory capacity of the extended memory.
The "EXTENDED memory" means a memory region which can be directly accessed by a CPU (central processing unit) in a protect mode. In addition to the "EXTENDED memory", an "EXPANDED memory" region is provided. The "EXPANDED memory" is indirectly accessed through a window area. In this description, only the "EXTENDED memory" will be described.
In FIG. 1, a 1 MB-extended memory 12 is coupled to a 1 MB-standard memory 11 having a 384 KB-reserve region. In this example, the reserve region of the standard memory 11 is assigned to memory addresses corresponding to 2 MB to 2 MB+384 KB.
In a conventional computer system, the capacity of the extended memory is fixed. Thus, memory addresses assigned to the reserve region of the standard memory are also fixed. However, in a computer system such as a lap-top type personal computer, extended memories of various capacities such as 1 MB, 2 MB, 3 MB, etc. have recently been used. In a computer system which can use such various extended memories, when the extended memories are used as "EXTENDED memory", the memory address assigned to the 384 KB reserve region in the standard memory needs to be changed from 2 MB to 2 MB+384 KB, 3 MB to 3 MB+384 KB, and 4 MB to 4 MB+384 KB, in accordance with the capacity of the extended memory.